Rescuers searched a damaged Iranian coal mine for a second day on May 4 as hopes of finding survivors faded following a blast, with the labor minister saying the death toll had risen to 35.

The explosion tore through the Zemestanyurt mine in Golestan Province on May 3, reportedly trapping up to 80 miners underground at a depth of about 1,800 meters.

Among those killed were workers who rushed to the aid of their colleagues but died when a tunnel collapsed, the semiofficial Fars news agency quoted Labor Minister Ali Rabiei as saying on May 4.

Iranian media gave conflicting accounts of how many people were in the mine at the time of the blast.

It was also unclear how many miners were injured, with local media reports on May 3 putting the number at 69.

State media did not immediately report on the rise in the death toll.

In a live broadcast by state television, Sadegh Ali Moghadam, the provincial director general of disaster management, said 22 bodies had been recovered.

Authorities said the blast occurred after a build-up of methane gas.

The province will observe three days of mourning for the victims of the explosion, Iranian state television reported.

President Hassan Rohani issued an order demanding his government use all available resources to rescue those still trapped, according to the state-run IRNA news agency.

Iran extracted 1.68 million tons of coal in 2016, which the Reuters news agency said was an increase on previous years resulting from the easing of international sanctions following a 2015 deal to curb Tehran's nuclear activities.

Iran uses most of the coal it mines in domestic steel production, exporting a relatively small amount.